feat: adaptation to btc signet

[EthanYuan]

The btc signet has its own special definition of difficulty consensus, such as powLimit, which is different from both maintain and testnet.

In order to support btc signet spv with spv type contract, this pr will be realized by extending flags. Flags is the u8 data at the end of the spv client cells args.

The expanded Flags are as follows：

// Constants for the chain type flag
// Specifically utilizing the two highest bits for chain type identification
pub const FLAG_CHAIN_TYPE_MAINNET: u8 = 0b0000_0000; // for mainnet
pub const FLAG_CHAIN_TYPE_TESTNET: u8 = 0b1000_0000; // for testnet
pub const FLAG_CHAIN_TYPE_SIGNET: u8 = 0b0100_0000; // for signet

Compatibility Notes:

• For spv instance for testnet updated with this pr, spv service does not need to be upgraded.
• But for spv instance for signet to be deployed and updated, service needs to be updated to support signet as well (new pr will be submitted to service later)

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Another modification is that POW should be checked regardless of chain type. Checking POW is the most important security for spv client on chain.

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• CI needs to wait for the following data to be uploaded before it can be passed through：

• chore: add signet spv update tx test data ckb-bitcoin-spv-testdata#7